1. Field the Invention
The present invention relates to an information signal encoding apparatus for decreasing the amount of generated data of digital information signals such as a digital audio signal and a digital picture signal, an encoding method thereof, and a decoding method thereof.
2. Description of Related Art
A variety of prediction encoding methods for decreasing the amount of transmission information such as a digital audio signal and a digital picture signal are known. As an example, in the one-dimensional DPCM method, the difference between an input sample value and a predicted value is formed in the temporal direction. In the two-dimensional DPCM method, the difference between an input sample value and a predicted value is formed in the spatial direction. Since the digital information signal has correlation in the temporal direction and the spatial direction, the levels of difference signals are lower than the sample values. Thus, since the difference signals are requantized with a smaller number of quantizing bits than the original number of quantizing bits, the amount of information can be compressed.
As an example of the quantizing apparatus using the difference signals, a nonlinear type quantizing apparatus is known. In the nonlinear type quantizing apparatus, the quantizing step width is finely divided at nearly 0 of the levels of the difference signals, whereas the quantizing step width is coarsely divided in proportion to the levels of the difference signals. In the conventional quantizing apparatus (including the nonlinear type quantizing apparatus) quantizes all levels of the difference signals. For example, when one sample (one pixel) of a digital picture signal is quantized with eight bits, the difference signals have values in the range from (-255 to +255). In other words, the conventional quantizing apparatus performs the quantizing process for all the range.
Since the conventional quantizing apparatus performs the quantizing process for all the range of the difference signals, when the number of quantizing bits is decreased, the quantizing accuracy is deteriorated. In contrast, when the number of quantizing bits is increased, the amount of information generated is proportionally increased. Thus, when data is decoded, the quality of the resultant audio signal and picture signal is not satisfactorily high.